Emplacement of the Paleoproterozoic Itareru Tonalite (NE São Francisco Craton, Brazil) during arc-continent collision revealed by magnetic fabrics.

Abstract

The Itareru tonalite stands as a remarkable Paleoproterozoic plutonic rock formation that intruded the Serrinha block, an ancient crustal fragment of the São Francisco Craton situated in Bahia, Brazil. This elongated, leaf-shaped body, stretching approximately 80 kilometers in a NW-SE direction, is predominantly composed of tonalite-diorite to granodiorite rocks, interspersed with numerous dioritic enclaves. Intriguingly, magnetic studies conducted on 26 sites within the Itareru pluton, including the surrounding country rocks, have revealed captivating insights into its magnetic fabric and the underlying geological processes that shaped its formation. Anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) measurements performed on rock samples from the Itareru pluton exhibit a remarkable degree of co-axiality. This observation suggests that all magnetic minerals within the rocks share a common orientation, which is partially attributed to the influence of regional strain. Further analysis of rockmagnetism data indicates that paramagnetic matrix minerals play a significant role in contributing to the overall magnetic susceptibility and AMS characteristics. However, for AARM, magnetite grains ranging from 2 to 5 micrometers in size are recognized as the primary contributors. In the central region of the pluton, magnetic fabrics exhibit signatures that are likely primary in origin, implying that these fabrics were acquired during the rocks' crystallization phase. This suggests that the magnetic fabrics reflect the flow of magma within the pluton, potentially linked to a magma feeder zone or magma chamber. The moderately inclined lineation observed in the northern part of the pluton suggests that this region was situated close to the feeder zone. In contrast, the horizontal to sub-horizontal lineations in the southern region indicate that this part of the pluton was farther away from the feeder zone. The emplacement of the Itareru tonalite was not only influenced by magmatic processes but also by regional strains associated with the collision of the Paleoproterozoic Rio Itapicuru greenstone belt with the Archean basement. This interplay between magmatic and tectonic forces resulted in the distinctive features of the Itareru tonalite, making it a prominent geological formation within the São Francisco Craton.